Improve detection of collection element type

src/JsonApiDotNetCore.Annotations/CollectionConverter.cs

@@ -104,17 +104,26 @@ public IReadOnlyCollection<IIdentifiable> ExtractResources(object? value)
     /// </summary>
     public Type? FindCollectionElementType(Type? type)
     {
-        if (type is { IsGenericType: true, GenericTypeArguments.Length: 1 })
+        if (type != null)
         {
-            if (type.IsOrImplementsInterface<IEnumerable>())
+            Type? enumerableClosedType = IsEnumerableClosedType(type) ? type : null;
+            enumerableClosedType ??= type.GetInterfaces().FirstOrDefault(IsEnumerableClosedType);
+
+            if (enumerableClosedType != null)
             {
-                return type.GenericTypeArguments[0];
+                return enumerableClosedType.GenericTypeArguments[0];
             }
         }
 
         return null;
     }
 
+    private static bool IsEnumerableClosedType(Type type)
+    {
+        bool isClosedType = type is { IsGenericType: true, ContainsGenericParameters: false };
+        return isClosedType && type.GetGenericTypeDefinition() == typeof(IEnumerable<>);
+    }
+
     /// <summary>
     /// Indicates whether a <see cref="HashSet{T}" /> instance can be assigned to the specified type, for example:
     /// <code><![CDATA[

src/JsonApiDotNetCore/Configuration/ResourceGraphBuilder.cs

@@ -396,8 +396,8 @@ private ReadOnlyCollection<EagerLoadAttribute> GetEagerLoads(Type resourceClrTyp
                 continue;
             }
 
-            Type innerType = TypeOrElementType(property.PropertyType);
-            eagerLoad.Children = GetEagerLoads(innerType, recursionDepth + 1);
+            Type rightType = CollectionConverter.Instance.FindCollectionElementType(property.PropertyType) ?? property.PropertyType;
+            eagerLoad.Children = GetEagerLoads(rightType, recursionDepth + 1);
             eagerLoad.Property = property;
 
             eagerLoads.Add(eagerLoad);
@@ -459,14 +459,6 @@ private static void AssertNoInfiniteRecursion(int recursionDepth)
         }
     }
 
-    private Type TypeOrElementType(Type type)
-    {
-        Type[] interfaces = type.GetInterfaces().Where(@interface => @interface.IsGenericType && @interface.GetGenericTypeDefinition() == typeof(IEnumerable<>))
-            .ToArray();
-
-        return interfaces.Length == 1 ? interfaces.Single().GenericTypeArguments[0] : type;
-    }
-
     private string FormatResourceName(Type resourceClrType)
     {
         var formatter = new ResourceNameFormatter(_options.SerializerOptions.PropertyNamingPolicy);

test/JsonApiDotNetCoreTests/UnitTests/TypeConversion/CollectionConverterTests.cs

@@ -0,0 +1,104 @@
+using FluentAssertions;
+using JsonApiDotNetCore;
+using Xunit;
+
+namespace JsonApiDotNetCoreTests.UnitTests.TypeConversion;
+
+public sealed class CollectionConverterTests
+{
+    [Fact]
+    public void Finds_element_type_for_generic_list()
+    {
+        // Arrange
+        Type sourceType = typeof(List<string>);
+
+        // Act
+        Type? elementType = CollectionConverter.Instance.FindCollectionElementType(sourceType);
+
+        // Assert
+        elementType.Should().Be<string>();
+    }
+
+    [Fact]
+    public void Finds_element_type_for_generic_enumerable()
+    {
+        // Arrange
+        Type sourceType = typeof(IEnumerable<string>);
+
+        // Act
+        Type? elementType = CollectionConverter.Instance.FindCollectionElementType(sourceType);
+
+        // Assert
+        elementType.Should().Be<string>();
+    }
+
+    [Fact]
+    public void Finds_element_type_for_custom_generic_collection_with_multiple_type_parameters()
+    {
+        // Arrange
+        Type sourceType = typeof(CustomCollection<int, string>);
+
+        // Act
+        Type? elementType = CollectionConverter.Instance.FindCollectionElementType(sourceType);
+
+        // Assert
+        elementType.Should().Be<string>();
+    }
+
+    [Fact]
+    public void Finds_element_type_for_custom_non_generic_collection()
+    {
+        // Arrange
+        Type sourceType = typeof(CustomCollectionOfIntString);
+
+        // Act
+        Type? elementType = CollectionConverter.Instance.FindCollectionElementType(sourceType);
+
+        // Assert
+        elementType.Should().Be<string>();
+    }
+
+    [Fact]
+    public void Finds_no_element_type_for_non_generic_type()
+    {
+        // Arrange
+        Type sourceType = typeof(int);
+
+        // Act
+        Type? elementType = CollectionConverter.Instance.FindCollectionElementType(sourceType);
+
+        // Assert
+        elementType.Should().BeNull();
+    }
+
+    [Fact]
+    public void Finds_no_element_type_for_non_collection_generic_type()
+    {
+        // Arrange
+        Type sourceType = typeof(Tuple<int, string>);
+
+        // Act
+        Type? elementType = CollectionConverter.Instance.FindCollectionElementType(sourceType);
+
+        // Assert
+        elementType.Should().BeNull();
+    }
+
+    [Fact]
+    public void Finds_no_element_type_for_unbound_generic_type()
+    {
+        // Arrange
+        Type sourceType = typeof(List<>);
+
+        // Act
+        Type? elementType = CollectionConverter.Instance.FindCollectionElementType(sourceType);
+
+        // Assert
+        elementType.Should().BeNull();
+    }
+
+    // ReSharper disable once UnusedTypeParameter
+    private class CustomCollection<TOther, TElement> : List<TElement>;
+
+    private sealed class CustomCollectionOfIntString : CustomCollection<int, string>;
+}